Improve the money section of the Organizer's Manual

[olasitarska]

Moved from Trello card:

• Make sure it's obvious that organizers need to find a way to legally handle money (through a non-profit or company)
• Point out that PayPal is dangerous
• Advise to ask conference organizers, if Django Girls is part of a conference
• General advise about taxes
• What to do with leftover money
• What to do if event is close and I don't have enough money